Catalytic reforming is a well established refinery process for improving the octane quality of naphthas or straight run gasolines. Reforming can be defined as the total effect of the molecular changes, or hydrocarbon reactions, produced by dehydrogenation of cyclohexanes, dehydroisomerization of alkylcyclopentanes, and dehydrocyclization of paraffins and olefins to yield aromatics; isomerization of substituted aromatics; and hydrocracking of paraffins which produces gas, and inevitably coke, the latter being deposited on the catalyst. In catalytic reforming, a multifunctional catalyst is usually employed which contains a metal hydrogenation-dehydrogenation (hydrogen transfer) component, or components, usually platinum, substantially atomically dispersed on the surface of a porous, inorganic oxide support, such as alumina. The support, which usually contains a halide, particularly chloride, provides the acid functionality needed for isomerization, cyclization, and hydrocracking reactions.
Reforming reactions are both endothermic and exothermic, the former being predominant, particularly in the early stages of reforming with the latter being predominant in the latter stages. In view thereof, it has become the practice to employ a reforming unit comprised of a plurality of serially connected reactors with provision for heating the reaction stream as it passes from one reactor to another. There are three major types of reforming: semi-regenerative, cyclic, and continuous. Fixed-bed reactors are usually employed in semi-regenerative and cyclic reforming, and moving-bed reactors in continuous reforming. In semi-regenerative reforming, the entire reforming process unit is operated by gradually and progressively increasing the temperature to compensate for deactivation of the catalyst caused by coke deposition, until finally the entire unit is shut-down for regeneration and reactivation of the catalyst. In cyclic reforming, the reactors are individually isolated, or in effect swung out of line, by various piping arrangements. The catalyst is regenerated by removing coke deposits, and then reactivated while the other reactors of the series remain on stream. The "swing reactor" temporarily replaces a reactor which is removed from the series for regeneration and reactivation of the catalyst, which is then put back in the series. In continuous reforming, the reactors are moving-bed reactors, as opposed to fixed-bed reactors, with continuous addition and withdrawal of catalyst. The catalyst descends the reactor in an annular bed and is passed to a regeneration zone and after regeneration it is recycled to the moving-bed reactor. Both the reactor and the regenerator are operated in a continuous moving-bed mode.
With the gradual phasing out of lead from the gasoline pool and with the introduction of premium grade lead-free gasoline in Europe and the United States, petroleum refiners must re-evaluate how certain refinery units are run to meet this changing demand for higher octane fuels with the use of lead. Because catalytic reforming units produce product streams which represent the heart of the gasoline pool, demands are being put on these units for generating streams with ever higher octane ratings.
Multistage reforming process schemes have been developed wherein the reactant stream is partially reformed at relatively low severity in a first stage. The partially reformed stream undergoes a separation between stages to remove the heavier aromatic components before they can be passed to a low pressure stage. This is because reforming reactors operated in semiregenerative and cyclic modes cannot operate with a full range first stage product stream at low pressure and low hydrogen/oil ratio typified by such processes. A full range product stream at such conditions would cause too much carbon, or coke, to deposit on the catalyst, thus leading to rapid catalyst deactivation.
U.S. Patents which teach such a process scenario are U.S. Pat. Nos. 4,872,967 and 4,975,178, which are incorporated herein by reference, and which teach a multistage process for semiregenerative and cyclic reforming wherein an aromatics-rich stream is separated from an aromatics-lean stream between stages. The aromatics-rich stream, which is high in octane, is collected for mogas blending stock and the aromatics-lean stream is passed for additional reforming in the next downstream stage, which is typically operated at higher severity, or lower pressures, than the upstream stage. This results in increased yields and hydrogen production over more conventional reforming processes, such as single stage semi-regenerative and cyclic processes.
While these two patents teach the aromatics-lean stream going to additional reforming, it is also taught in copending application U.S. Ser. No. 747,894; filed Aug. 19, 1991. that there can be a benefit gained if, after heavier aromatics separation between stages, the stream, which is passed to the next downstream stage for additional reforming, contains a substantial amount of lighter aromatics. The stream passed to the next downstream stage in that application is a stream comprised of C.sub.8 and lower carbon number aromatics, as well as uncoverted paraffins. C.sub.9.sup.+ aromatics are removed by distillation.
Also, U.S. Pat. No. 3,992,465 teaches a two stage reforming process wherein the first stage is comprised of at least one fixed-bed reforming zone and the second stage is comprised of a moving-bed reforming zone. The teaching of U.S. Pat. No. 3,992,465 is primarily to subject the reformate, after second stage reforming to a series of fractionations and an extractive distillation of the C.sub.6 -C.sub.7 cut to obtain an aromatic-rich stream.
While such teachings are a step in the right direction, there still remains a need in the art for improved reforming processes which can overcome such disadvantages. There is also a need in the art for the modification of conventional fixed-bed reforming process units to incorporate some of the advantages of moving-bed reforming units, without having to build an entirely new grass-roots moving-bed unit.